Malignant mesotheliomas are notoriously difficult to distinguish, by the existing morphological and biochemical methods, from many other malignant tumors which often invade serious cavities. I have prepared an anti-mesothelial cell specific antiserum which reacts specifically with cells of mesothelial origin. The specificity of the antiserum had been demonstrated by immunofluorescence and immunoperoxidase staining and by immoundiffusion-precipitin reaction with the extracts of various tissues and tumors. I propose to isolate the mesothelial-specific antigen by affinity chromatography. The character of the antigen will be studied by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Using the addinity purified antigen, a high titered antiserum will be raised and a sensitive immunoassay for quantitation of mesothelial cell specific antigen in biological fluids will be established. The assay will be a modification of the enzyme linked immunosorbent assay. The quantitative assay for mesothelial cell specific antigen in serum will be evaluated as a non-invasive diagnostic test for malignant mesothelioma. The assay will also be used as a means of quantitating the mesothelial cell mass, to assess the response to treatment, and to prognosticate. It may, at a later date, be possible to use the antiserum in a site-directed chemotherapy program.